(Working at SATP - Standard Ambient Temperature and Pressure; pressure of 100.00 kPa and a temperature of 25 °C).
Laboratory Method:
Da Bomb Calorimeter is used to measure the amount of energy gained or lost in a chemical reaction.
*This works since energy lost in an exothermic reaction (where ΔH is negative) is equal to energy gained by water (Q). N.B. Signs are opposite. If it is an exothermic reaction (-ΔH), then energy is gained by water (+).
Graphically: ∆Hr = ∑ potential energy of products - ∑ potential energy of reactants
In practice, at STP, we look up the heat of formation (which is potential energy). ∆H reaction = ∑H formation (products) - ∑ H formation (reactants)
Rules:
1) There is no heat of transformation for elements.
2) Diatomic elements (HOFBrINCl) or polyatomic elements (S) have no heat formation.
Example:
2H2 + O2 → 2H2O
(According to rule 2 above, 2H2 has 0 heat formation and so does O2 since they are diatomic).Therefore, ∆Hr = ∑Hf(products) - ∑ Hf (reactants)
∆Hr = 2[∆H formation (H2O)] - 0
∆Hr = 2[∆H formation (H2O)]
In this reaction, heat of reaction in the molecules is the heat of formation of molecule.
Other method to calculate ∆Hr
We can also calculate ∆Hr (not be standard; however, using the energy of bonds). At STP, we will always get a specific value but with this method, it would be an estimate since same type of bond will have different energy according to where it is attached. Ex.
Methanol Water
The O-H bonds in methanol will haev different energy than O-H bonds in Water (it depends on the environment; can be due to polarity and other factors). 2H2 + O2 → 2H2O
N.B. ∆Hr = ∑Bonds energy of reactants - ∑Bonds energy of products
∆Hr = 2 (H-H) + 1(OO) - 4(H-O)
To break bond, input of energy is required.
To form a bond, energy is released.
Hess’ Law
Chemical reactions can be added together, and so can their heat of formation.
Example: Summation Reaction for Light Reaction (photosynthesis):
12 (H2O + λ + NADP + ADP → ½ O2 + NADPH2 + ATP) Dark Reactions:
6 CO2 + 12 NADPH2 + 12 ATP → glucose (C6H12O6) + 6 H2O + NADP + ADP Final Reaction:
6CO2 + 6H2O → glucose + 6O2
What Hess’ Law says is that if we add two reactions together
∆Hr = ∆Hf(A) + ∆Hf (B)
Calculating Heats of Reactions
(Working at SATP - Standard Ambient Temperature and Pressure; pressure of 100.00 kPa and a temperature of 25 °C).Laboratory Method:
Da Bomb Calorimeter is used to measure the amount of energy gained or lost in a chemical reaction.
*This works since energy lost in an exothermic reaction (where ΔH is negative) is equal to energy gained by water (Q).
N.B. Signs are opposite. If it is an exothermic reaction (-ΔH), then energy is gained by water (+).
Graphically:
∆Hr = ∑ potential energy of products - ∑ potential energy of reactants
In practice, at STP, we look up the heat of formation (which is potential energy).
∆H reaction = ∑H formation (products) - ∑ H formation (reactants)
Rules:
1) There is no heat of transformation for elements.
2) Diatomic elements (HOFBrINCl) or polyatomic elements (S) have no heat formation.
Example:
2H2 + O2 → 2H2O
(According to rule 2 above, 2H2 has 0 heat formation and so does O2 since they are diatomic).Therefore,
∆Hr = ∑Hf(products) - ∑ Hf (reactants)
∆Hr = 2[∆H formation (H2O)] - 0
∆Hr = 2[∆H formation (H2O)]
In this reaction, heat of reaction in the molecules is the heat of formation of molecule.
Other method to calculate ∆Hr
We can also calculate ∆Hr (not be standard; however, using the energy of bonds). At STP, we will always get a specific value but with this method, it would be an estimate since same type of bond will have different energy according to where it is attached.
Ex.
Methanol Water
The O-H bonds in methanol will haev different energy than O-H bonds in Water (it depends on the environment; can be due to polarity and other factors).
2H2 + O2 → 2H2O
N.B.
∆Hr = ∑Bonds energy of reactants - ∑Bonds energy of products
∆Hr = 2 (H-H) + 1(OO) - 4(H-O)
To break bond, input of energy is required.
To form a bond, energy is released.
Hess’ Law
Chemical reactions can be added together, and so can their heat of formation.
Example:
Summation Reaction for Light Reaction (photosynthesis):
12 (H2O + λ + NADP + ADP → ½ O2 + NADPH2 + ATP)
Dark Reactions:
6 CO2 + 12 NADPH2 + 12 ATP → glucose (C6H12O6) + 6 H2O + NADP + ADP
Final Reaction:
6CO2 + 6H2O → glucose + 6O2
What Hess’ Law says is that if we add two reactions together
∆Hr = ∆Hf(A) + ∆Hf (B)